Aspects of direct deduction of ground impedance

ANSI S1.18 1998 offers a method for determining ground impedance by fitting data to templates of the magnitude of level difference obtained using a point source and two vertically separated microphones with specified geometries according the ‘‘acoustical‐softness’’ of the ground. The ANSI working group on ground impedance is seeking to revise the current template method for deducing ground impedance by developing a practical method, based in fitting complex level difference spectra and without assuming any particular impedance model. Both minimization and root finding techniques have been used in published laboratory experiments. The results of trial outdoor measurements in the UK, USA, Germany, and Canada are presented. Issues have arisen concerning phase matching of microphones, the signal processing instrumentation, the site and meteorological constraints, and the smoothness of the resulting data.